JP 2007-311467 A discloses a printed board having a wiring fuse that is provided by a part of a wiring pattern. Hereinafter, the wiring fuse provided by a part of the wiring pattern is also referred to as an interrupting portion of the wiring pattern.
The interrupting portion has a width smaller than a width of a remaining portion of the wiring pattern. Hereinafter, the remaining portion of the wiring pattern is a portion other than the interrupting portion in the wiring pattern. Thus, for example, when an overcurrent that is caused by a short circuit occurred in an electronic component flows through the wiring pattern, the interrupting portion melts down by heat generated by the overcurrent and the overcurrent is interrupted.
Usually, the wiring pattern is formed by coating a conductive paste on a surface of a board. Specifically, the wiring pattern is formed by patterning a metal foil or by a screen printing. Usually, the metal foil has a variation in a thickness. Thus, when the wiring pattern is formed by patterning the metal foil, the interrupting portion of the wiring pattern has a variation in a thickness. Further, when a plating is carried out on the metal foil, a variation in a plating thickness is greater than the variation in the thickness of the metal foil. Thus, the interrupting portion has a greater variation in the thickness. When the wiring pattern is formed by the screen printing, a variation in an applied pressure during the screen printing causes a variation in a coating thickness, and the variation in the coating thickness causes the variation in the thickness of the interrupting portion. As well known, an interrupting performance of the interrupting portion is substantially affected by the variation in the thickness of the interrupting portion.
Conventionally, the interrupting performance of the interrupting portion is assured by inspecting a width of the interrupting portion with an optical visual inspection apparatus. Thus, it is hard to assure the interrupting performance of the interrupting portion with a high precision. Further, the thickness of the interrupting portion is hard to be inspected by the visual inspection apparatus. In order to inspect the thickness of the interrupting portion, an additional thickness measuring apparatus may be used to measure thicknesses at multiple points of the interrupting portion. However, the thickness measuring apparatus needs a high cost and the inspection process needs a longer time.